You can learn on a permit aeroplane if you sole-own it.
I'd argue learn on microlights if you want to fly microlights, and the NPPL(M) is an inefficient route to SEP/SSEA. However, the minimum hours to convert from NPPL(M) to NPPL(SSEA) is very small so that option is readily and cheaply available for anybody who has significant hours in 3-axis microlights in the UK.
Microlights are absolutely not an efficient route the JAR-PPL(SEP), you get 10% of your P1 hours off, up to 10 hours! Frankly this is daft, since anybody with 100hrs PiC of 3-axis microlights really should not not 35 hours to get a PPL(SEP); you can improve your value for money by getting the night qualification within that 35 hours.
G